Is it possible to achive classic Z depth type fog or mist in the viewport (3D-view) in Blender 2.8?
Since you're asking about the viewport, looks like volumetrics is the only option for now. It's very fast though and easy if you attach the volume scatter to the world shader.


World tab, check "use nodes"
Shader (Node) editor, select world instead of object
Add volume scatter (try density between 0.01 and 0.1) and plug into volume world node.


Doesn't require volumetrics to be enabled in eevee tab; works better viewing with camera. A super-fast hack in the compositor, at least fast in terms of cpu time, using z buffer (z render output -> divide by a two to  four digit number, add a number between 0 and 1, mix with image render output using lighten or screen) doesn't affect the viewport and doesn't work with DOF.

Good luck!